DST-3, a Novel Modified Cryptotanshinone, Protects Against Pulmonary Fibrosis via Inhibiting STAT3/Smad Signaling Pathway and Improves Bioavailability

<b>Background/Objectives</b>: Idiopathic pulmonary fibrosis (IPF) is a fatal lung disease characterized by progressive loss of lung function and poor prognosis. Cryptotanshinone (CTS), a small-molecule compound extracted from Salvia miltiorrhiza, possesses diverse pharmacological activities but suffers from poor oral bioavailability, which restricts its clinical development, particularly in pulmonary fibrosis. DST-3, a newly synthesized derivative of CTS, was designed to overcome these limitations. <b>Methods</b>: The antifibrotic effects of DST-3 were investigated in a bleomycin-induced pulmonary fibrosis model in C57BL/6 mice through lung function assessment, histopathological evaluation, hydroxyproline quantification, and cytokine profiling. In vitro, TGF-β1-stimulated MRC5 fibroblasts were employed to explore the mechanism of action, focusing on STAT3/Smad signaling via Western blotting and molecular binding assays. Furthermore, a validated HPLC-MS/MS method was developed for DST-3, and its pharmacokinetic profile was characterized in Sprague-Dawley rats and compared with that of CTS. <b>Results</b>: DST-3 markedly attenuated pulmonary fibrosis in vivo, as evidenced by improved lung function, reduced collagen deposition, and decreased proinflammatory cytokine levels. In vitro, DST-3 inhibited TGF-β1-induced fibroblast activation by directly binding to STAT3 and suppressing STAT3/Smad signaling. Pharmacokinetic analysis demonstrated that, compared with CTS, DST-3 exhibited more rapid absorption, a higher peak plasma concentration, a greater area under the curve (AUC), improved hepatic metabolic stability, and enhanced lung tissue exposure. <b>Conclusions</b>: Our study demonstrates that DST-3 exerts potent antifibrotic effects in vivo and in vitro, primarily through STAT3 pathway inhibition. Its improved pharmacokinetic characteristics further support its potential as a promising candidate for the treatment of pulmonary fibrosis.